Basically, it is a necessary evil.
Back during Fallout 3, many people complained that they could not play on after the main quest. A few of us thought this was the way it should be but we were a small insignificant minority. So BGS made a change that allowed people to play on, and on, and on, and on after the main quest was completed.
When BGS developed Fallout 4, they planned for people playing on after the main quest. The also decided to make the experience somewhat rewarding and allow players to continue to develop their characters. This is why we have the leventy million perks and no limit on character levels.
Now, as most of us know. In order to go out adventuring, we need supplies. We need a constant supply of ammunition, chems, food, water and for those of us with a desire to craft things, we need a whole host of crafting supplies.
In order to give us the required supplies, one of two things must be done. The first is that we would need a map large enough to accommodate adventuring for an indefinite period of time with a near infinite supply of consumables. If you know anything about Science! you already realize that the map would have to be HUGE. To accommodate the map, the Earth would have to be MASSIVE. So massive, that it would collapse into a hairy black hole. This implosion would destroy the entire Fallout universe and due to the millions of connections it has with ours, it would destroy our universe as well.
Instead of destroying two universes, BGS decided to use the second option and we have loot that respawns.
Now we can debate whether BGS chose the correct option, but ultimately it is their game and their choice.
Personally, I am kind of happy that I didn't cease to exist the moment I clicked "Play"